Quartz crystal microbalances (“QCM”) have been developed as sensitive chemical and biochemical sensing devices and can be used for the detection of disease-related particles such as viruses and bacteria in liquid samples (see e.g. Thompson, M. et al., Analyst Vol. 116, pp. 881–890, 1991; Rickert, J. et al., Biosensors & Bioelectronics Vol. 12, pp. 567–575, 1997; Uttenthaler, E. et al., Biosensors & Bioelectronics 16, 735–743, 2001). In this technology, a binding partner such as an antibody is attached to the surface of a small resonant quartz crystal with a mechanical resonance frequency typically in the 10 to 30 MHz region. If a disease-related particle binds to the antibody, the resonance frequency of the quartz crystal shows a very small shift, whereby such shift in frequency or a correlated phase shift between the electrical excitation and the mechanical vibration is an indication that an antibody-specific binding partner was present in the liquid sample.
A significant improvement in the detection sensitivity of a QCM biosensor has been achieved by applying the technology of rupture event scanning (“REVS”), (see Dultsev, F. N. et al., Langmuir Vol. 16, 5036–5040, 2000; Cooper, M. A. et al., Nature Biotechnology Vol. 19, 833–837, 2001; WO 01/02857 A1 to Klenerman et al.). In the REVS technology, as in the classic QCM technology, a binding partner such as an antibody is attached to the surface of a small resonant quartz crystal with a mechanical resonance frequency typically in the 10 to 20 MHz region. The liquid sample containing bacteria or viruses is brought into contact with the activated crystal surface so that binding events can take place.
After a 30-minute incubation period, the resonant quartz crystal is operated as close as possible to the fundamental mechanical resonance frequency, whereby the driving power for the quartz crystal is monotonously increased, until suddenly the binding between the binding partners is broken up. According to the inventors of REVS, such breaking or “rupture” event can be detected due to the emission of noisy sound waves with a preferred frequency spectrum around the third harmonic of the fundamental resonance frequency. The quartz crystal acts as a sensitive microphone, and the generated electrical signal is monitored via an electric resonance circuit tuned to a frequency close to the third harmonic of the fundamental resonance frequency of the crystal. The REVS technology has the potential of detecting the breaking-away of only a few binding partners, in other words, the technology offers the potential for extreme sensitive detection.
It is very likely that rupture events may also cause even stronger sound signals at or close to the fundamental resonance frequency of the quartz crystal, but due to the high-level driving signal in this frequency region, the weak REVS signal is obscured and can not be detected in prior art detection devices. As mentioned above, the inventors of REVS have, therefore, designed a detection setup where the REVS signal is coupled from the quartz crystal into a parallel resonance circuit tuned to a frequency close to the third harmonic, and from there into a narrow-band electronic receiver. The detection setup as disclosed in WO 01/02857 A1 is shown schematically in FIG. 1.
In the detection setup shown in FIG. 1, G1 is the driving signal generator working at the crystal's fundamental resonance frequency, F. QC is the vibrating quartz crystal, and G2 is a local oscillator working at a frequency 3F+ΔF with ΔF being a frequency offset of approximately 80 kHz. Generator G2 tunes the lock-in amplifier to the detection frequency 3F+ΔF. The quartz crystal QC is connected with the input of the lock-in amplifier via a parallel resonance circuit, tuned also to the frequency 3F+ΔF.
The quartz crystal can, near the third harmonic of its fundamental series resonance frequency, be modeled as a signal source for REVS signals with an internal ohmic impedance of about 50Ω. The ohmic impedance of the parallel resonance circuit, on the other hand, may be as high as 500 kΩ. This substantial impedance mismatch between the “source” and the front-end of the detection circuitry results in a non-optimized extraction of signal power from the REVS crystal. FIG. 2 illustrates how the detected power depends on the impedance matching between a 50-Ω source and the detector input impedance. From this figure it can be seen that a parallel resonance circuit with an effective ohmic component of 500 kΩ may not be an optimum front-end for a REVS detection setup.
In view of the disadvantages in prior art REVS-based acoustic detectors as described above, there exists still a need for a more optimized detection setup in an acoustic detector based on rupture event scanning.